The theoretical basis of tumor immunotherapy is that the immune system can identify tumor-associated antigens and regulate the body to attack tumor cells (highly specific cytolysis). In the 1950s, Burnet and Thomas made the theory of “immunological surveillance” that holds that mutational tumor cells that often occur in the body can be identified and eliminated by the immune system, laying a theoretical foundation for tumor immunotherapy [Burnet F M. Immunological aspects of malignant disease. Lancet, 1967; 1: 1171-4]. Then, a host of tumor immunotherapies, including cytokine therapy, monoclonal antibody therapy, adoptive immunotherapy and vaccine therapy, have been applied to clinical practice.
In 2013, CAR-T, a more advanced tumor immunotherapy, was successfully put to clinical use, and showed unprecedented clinical effects. CAR-T is short for Chimeric Antigen Receptor T-Cell Immunotherapy. Clinically, the most leading CAR-T is Novartis' CLT019. For patients with refractory-relapsed acute lymphoblastic leukemia and treated with CLT019, the six-month tumor progression-free survival rate can reach 67%, and the longest response time can be more than two years. By cooperating with hospitals, Shanghai Unicar-Therapy Bio-Medicine Technology Co., Ltd., a Shanghai-based company, treated 36 patients with refractory-relapsed acute lymphoblastic leukemia, among whom 24 as a percentage of 66.6% experienced complete remission. It's a subversive breakthrough in anti-cancer research. CAR-T may be one of the therapies that are the most likely to cure cancer, and was named the best in top 10 breakthroughs of science and technology 2013 by the journal Science.
Although CAR-T therapy is effective, it encounters many difficulties in the treatment of solid tumors. One of the important reasons is PD1/PDL1 immunosuppressive check points (as shown in FIG. 1A), which combine with the transmission of suppressive signals, inhibit the immunological activity of T cells, play an important role in immunological tolerance, and also promote the escape of tumor cells.
PD-1 (also known as CD279) is an immunosuppressive receptor, belonging to type I transmembrane protein of CD28 family members. Programmed cell death molecule-1 receptor was obtained and named by Ishida et al. in 1992 [Ishida Y, Agata Y, Shibahara K, et al. Induced expression of PD-1, a novel member of the immunoglobulin gene superfamily, upon programmed cell [J]. EMBO J, 1992, 11 (11): 3887-3895.] in apoptotic T cell hybridoma by subtractive hybridization. Human PD-1 gene is located on chromosome 2q37.35 and encodes a transmembrane glycoprotein of about 55 kD. PD-1 is widely expressed on the surface of activated T cells, B cells, monocytes and dendritic cells. The structure of PD-1 shares 30% homology with CTLA-4. There are two tyrosine residues in the intracellular domain, which are involved in the formation of an immunereceptor tyrosine-based inhibitory motif (ITIM) at the N-terminal and an immunoreceptor tyrosin-based switch motif (ITSM) at the C-terminal respectively. The extracellular domain consists of an IgV-like domain, which contains multiple glycosylation sites and is heavily glycosylated. The domain can bind to ligands, thus exerting the function of inhibiting T cell activation [Li Ying, Jiao Shunchang, et al. The role and clinical significance of PD-1/PD-L1 signaling pathway in tumor immune escape [J]. Acad J Chin PLA Med Sch, July 2015, 36 (7)].
PD-L1 is overexpressed in most cancer tissues, including NSCLC, melanoma, breast cancer, glioma, lymphoma, leukemia and various urinary, digestive and reproductive tumors [Intlekofer A M, Thompson C B. At the bench:preclinical rationale for CTLA-4 and PD-1 blockade as cancer immunotherapy[J]. J Leukoc Biol, 2013, 94(1):25-39.]. Parsa found that abnormal IFN-γ secreted by T cells in tumor cells of rats and humans could induce high expression of PD-L1 [Ding H, Wu X, Wu J, et al. Delivering PD-1 inhibitory signal concomitant with blocking ICOS co-stimulation suppresses lupus-like syndrome in autoimmune BXSB mice[J]. Clin Immunol, 2006, 118(2/3):258-267. The high expression of PD-L1 can regulate the expression of cell cycle check point protein and cell proliferation-related protein by inhibiting RAS and PI3K/AKT signaling pathway, and ultimately lead to the inhibition of T cell proliferation [11]. Dong et al. in vitro experiments and mouse models also found that activation of PD-1/PD-L1 signaling pathway can induce specific CTL apoptosis, reduce the sensitivity of CTL to cytotoxicity and induce immune escape of tumor cells [Dong H, Strome S E, Salomao D R, et al. Tumor-associated B7-H1 promotes T-cell apoptosis: a potential mechanism of immune evasion[J]. Nat Med, 2002, 8(8): 793-8001
At present, commercial PD1 monoclonal antibodies are mainly used as immunological check point inhibitors to inhibit the immune escape of cancer cells. On Sep. 3, 2014, Opdivo (Nivolumab), an anti-PD-1 drug from Bristol-Myers Squibb, was officially launched in Japan. The drug is still limited to melanoma patients in Japan. The drug is set at the price of 729849 yen (about 43000 yuan) per 100 mg. Because each 1 kg need 2 mg, people who weigh 50 kg need 100 mg. In addition, every 10 kg increase in body weight required an increase of 20 mg (150200 yen, about 8778 yuan), every three weeks for a course of treatment. The price is so expensive that ordinary families can't afford it.
Therefore, how to use low-cost methods to inhibit the occurrence of immune escape of cancer cells without affecting the efficacy of CAR-T treatment has become a technical problem of CAR-T treatment.